Now he is on the four-person Senate trial witness wish list issued by Minority Leader Chuck Schumer on behalf of Senate Democrats. (The other three are acting White House chief of staff Mick Mulvaney, Office of Management and Budget official Michael Duffey and White House aide Robert Blair, who is Mulvaney's national security adviser.)

2. It's not what Senate Republicans want

Under the leadership of Majority Leader Mitch McConnell, Republicans have largely opposed the idea of witnesses at a Senate impeachment trial. Even those like Maine's Sen. Susan Collins and Alaska's Sen. Lisa Murkowski, who have expressed some frustration with the process, have not definitively said they would support voting with Democrats for new testimony at an impeachment trial. 

So Bolton is volunteering to do something there's a very good chance he'll never have to do. But in his statement he refers to his "obligations as both a citizen and as former National Security Adviser," as if they are in conflict.

3. He has clashed with Trump

He's been happy and quite complimentary on Twitter of Trump's combative new approach to Iran. But the idea that a former staffer is teasing that he's got a story to tell can't be what the White House or Trump wants to hear.

4. This is the second time he has intentionally reinserted himself into the impeachment drama

Now Bolton says he won't need to wait for the courts. After careful consideration, and since the courts won't be weighing in before a Senate trial, he's "prepared to testify." 

Reminder: He's got a book coming out

But what might he say?

Hill, who reported to Bolton at the National Security Council, testified about his disapproval of Trump's efforts to pressure Ukraine to investigate former Vice President Joe Biden and his son Hunter's relationship with Burisma, a Ukrainian natural gas company.

Will he get a chance to say it?

Democrats are saying -- They've argued that Bolton's statement shows Nancy Pelosi's strategy of withholding the articles of impeachment from the Senate in hopes of getting a more fair trial is working. Remember, it takes four Republicans voting with Democrats to compel testimony. 

Schumer promised Monday to force votes on witness testimony. "A trial without all the facts is a farce," he said, seeding the idea that acquittal will be meaningless without witness testimony. "The verdicts of kangaroo court are empty."

Republicans are saying -- Sen. Shelley Moore Capito, a West Virginia Republican, would not say if she would vote to subpoena Bolton or if she wants to hear testimony from him. "I'm just going to wait and see what happens," she said.

This is in line with McConnell's preference that senators hear arguments about impeachment first and then decided whether to call witnesses.

Sen. Susan Collins of Maine, who is facing a tough reelection campaign, said she's open to witnesses later on, after arguments and after senators have a chance to ask questions.

"There are a number of witnesses that may well be appropriate," she said.

Another endangered Republican, Sen. Martha McSally of Arizona, just said senators are working through all of this.

The opposition force -- Sen. Marco Rubio of Florida, a member of the Senate Intelligence Committee, told CNN he would not vote for a subpoena of Bolton because it was the House of Representatives' job to get his testimony.

"If the House wants to start a new impeachment inquiry or pull it back and add additional elements to it, that's their choice to make," he said. 

Ahem, Sen. Rubio. Two previous presidential impeachments featured fresh witness testimony in the Senate trial.

Protecting the Senate from a 'circus' -- Sen. John Cornyn of Texas, a member of the Republican leadership team, evolved over the course of the day, according to CNN's Phil Mattingly and Ted Barrett.

Cornyn initially told reporters he "has no objection" to Bolton testifying "by deposition or some prerecorded testimony."

But he later indicated he would not vote to subpoena Bolton and warned against a "circus-like atmosphere" like the one he believes there was in the House. 

Cornyn also repeatedly said he thought Bolton's testimony would be stifled by executive privilege, which he said would likely be invoked by Trump on much of what Bolton would have to say. 

On the podcast

The topic of Bolton and how the issue of witnesses affects a Senate trial took over the Impeachment Watch Podcast on Monday. David Chalian and I talked to Alice Stewart, the GOP strategist and CNN analyst, about whether she'd advise purple-state Republicans to vote with Democrats on the issue.

Trump talks impeachment with Rush Limbaugh

"It's so sad for our country," Trump told the conservative radio host during a phone-in interview. "We're fighting with Iran, we're fighting with all of these different places -- and in many cases doing great, making trade deals and doing so good -- but I have to spend, and my team has to spend, time on this stuff."

Asked what he thought was behind Speaker Nancy Pelosi's decision to withhold sending the articles of impeachment to the Senate, Trump responded: "Well, I think what they're trying to do is affect the election, illegally, but that's what they're trying to do. The reason they're not sending them is because they're -- they are a joke. They are not crimes. There's nothing there."

What we know won't happen

Sometimes nothing is as refreshing to read as "we don't know." 

To questions like "When will the trial begin?" Mattingly is not afraid to say we don't know. Because we don't. 

One thing he says we do know is that McConnell will not attempt to change Senate rules to proceed with a trial before Pelosi officially transmits the articles of impeachment. 

Why? It would effectively end the legislative filibuster and McConnell has shown no interest in doing that.

What are we doing here?

The President has invited foreign powers to interfere in the US presidential election. Democrats impeached him for it. A Senate trial is next. It is a crossroads for the American system of government as the President tries to change what's acceptable for US politicians. This newsletter will focus on this consequential moment in US history.